The chargeable lithium-ion battery has been used widely in portable devices. Currently, each large chip manufacturing company has its own dedicated lithium-ion battery charging chips to charge a lithium-ion battery formed by single or multi cells in series. Currently, most of the charging solutions use BUCK type topology, as shown in FIG. 1. The circuit has simple structure and high efficiency.
To further improve the efficiency, synchronous rectification technology is more and more widely used in charging circuits. FIG. 2 is a BUCK type charging circuit using synchronous rectification technology. For the charging circuit of FIG. 2, the switch S1 and the diode D2 shown in FIG. 1 are replaced with MOSFET respectively, which effectively reduces a the on-state loss caused by a positive on-state voltage drop of a diode and usually achieve an efficiency of more than 90%. Moreover, in order to further reduce the volume and the number of elements for the charging circuit, some charging management chips have integrated the switch devices inside the chip.
Since the battery is different from the common loads and is equivalent to a direct current supply, when the BUCK type synchronous rectification circuit charges the lithium-ion battery formed by the multi battery cells in series, the input voltage of the charging circuit is higher than that of the charged battery because the voltage of the charged battery is higher. At that time, the current of the battery will reflect to the input of the charging circuit if the anti-reflect MOSFET Q3 and the charging circuit can not be cut off in time when the direct current input powers down or voltage sag occurs. The energy of the inverse current may be from the charged battery or the output energy storage capacitance C2, and the reflected inverse current flows through L1. Here, if the synchronous rectification circuit is not cut off, the synchronous rectification circuit will keep working. In the case that there is an inverse current appearing in an inductance, if the synchronous rectification circuit keeps working, it will cause the synchronous rectifier Q2 to be short-circuit relative to the ground during the synchronous continuous current. A too large short-circuit current will impair Q2 instantaneously, which thereby further cause impairments to Q1 and Q3. If Q1˜Q3 are integrated inside the charging chip, it will directly damage the chip. Therefore, the anti-reflection MOSFET and the charging circuit must be cut off immediately after the input voltage is falling, to prevent the charging circuit from being damaged when the voltage reflected.
Currently, there are two ways to cut off the charging circuit in time:
1) A diode or a control switch is inserted serially to the main loop of the charging circuit, as shown in the solution realized by D1 of FIG. 3 and FIG. 4, which prevents battery voltage reflection, however, this method lead to low efficiency due to a large positive voltage drop of the diode. In order to improve the efficiency, a MOSFET (i.e. Field Effect Transistor) is added in parallel to the anti-reflection diode. Although this method is reliable to accomplish cutting off the diode automatically, it cannot achieve high efficiency because of a high positive voltage drop of the diode, and it is difficult to make the miniaturization and modularization of the charging circuit because of the heating generated by the diode.
2) The method for comparing the battery voltage and the input voltage is applied. As shown in FIG. 5, when the input voltage is ΔV higher than the battery voltage and ΔV is less than 75 mV, the control circuit will enter to a sleeping mode in which the anti-reflection MOSFET and the charging circuit are cut off, and once ΔV is larger than 75 mV, the entire circuit will be waken up, wherein the anti-reflection MOSFET is turned on again and the synchronous rectification circuit keeps working. However, the battery voltage and the input voltage applied in this method are both floating voltage and the comparing threshold is only 75 mV. Since the battery voltage and the input voltage can not be judged correctly because the battery voltage and the input voltage both have ripple when they are working, and a sampling circuit of the battery voltage has certain delay, this method can not provide a effective cutting-off to the anti-reflection MOSFET and the charging circuit in time which makes it less reliable, thus to increase the risk that the anti-reflection MOSFET Q3 and the charging circuit are damaged. In practical, this risk increases with the increase of the number of the charged batteries that are connected serially.